The present invention relates to a white light emitting device.
FIG. 25 is a sectional view of a conventional LED device disclosed in U.S. Pat. No. 6,069,440. The white LED device 70 comprises a substrate 73, electrodes 71 and 72 secured to the substrate 73 and a blue LED 61 mounted on the substrate 73. The LED 61 is connected to the electrodes 71 and 72 by lead wires 62. The LED 61 and electrodes 71, 72 are encapsulated with a transparent encapsulating resin 91.
In the resin 91, fluorescent materials of YAG group are mixed. The fluorescent material comprises phosphor particles 81.
When the current is applied to the blue LED 61 via electrodes 71 and 72, the LED emits blue light Pb. When a portion of the blue light impinges upon the phosphor particle, the phosphor particle absorbs the blue light and emits yellow light Py. When the yellow light Py and the blue light Pb are combined, white light Pw is created.
Furthermore, U.S. Pat. No. 6,319,425 discloses an LED covered by a cap in which fluorescent material is included.
Further, U.S. Pat. No. 6,351,069 discloses an LED encapsulated by a transparent resin in which two kinds of phosphor particles are included, thereby producing white light.
However, since the LED is a compound semiconductor, there is wide variation in chromaticity and luminance of products. Further, the chromaticity and luminance of the mixed white light Pw also vary widely because of difference of quantity and distribution of the phosphor particles in the encapsulating resin.
FIGS. 26 and 27 are graphs showing numeric data widely varied in chromaticity and luminance of LED devices.
FIG. 26 is a graph of XYZ chromaticity coordinates showing variation in chromaticity of LED devices by 1 lot mass production. Each black point indicates the chromaticity of an LED device. The variation is in an upward zonal arrangement. Here, the dispersion of the black points in the width direction shown by the letter A means the variation of chromaticity of the blue LED, the dispersion in the longitudinal direction shown by the letter B means the variation of quantity and distribution of the phosphor particles in the encapsulating resin.
An LED device having chromaticity which is largely deviated from a central value in FIG. 26 can not be used as a device for emitting white light. It is frequently desirable that the LED device has chromaticity in a central range shown by hatching where X and Y are in a range of 0.33±0.01.
In the graph of FIG. 27, the horizontal axis shows luminance and the vertical axis shows the number of LED devices. The luminance variation is distributed in +30% to −40% range from the center of the distribution. However, a desirable range R2 is about ±20% as shown by arrows in FIG. 27.